To start with, I don't believe there is a single cause for infidelity. It, like sex, is a complex behavior that has a multitude of causes. But, there is building evidence that the hormone Testosterone (called "T") is behind a lot of the physiological and neurological processes that increase the likelihood of infidelity. We know already that men whose fingers, jaws and behaviors (such as assertive and dominant behaviors in social interactions) are more likely to admit that they have engaged in infidelity at some point in their lives. These effects are related to prenatal levels of T that affected a child's development in utero. But, fascinating research suggests that current T levels also predict several things related to men (and women's) sexual behaviors and relationships.
Sari van Anders, Ph.D. is a Michigan psychologist who has done fascinating assessments of T levels in men and women, looking at their relationship status, and their reports of sexual partners inside and outside their marriages. In 2006, she found that high levels of T predict whether both men will become and remain partnered in a relationship. Higher levels of T make it less likely that people will commit to a relationship. When they do, they are more likely to have relationships that are nonmonogamous, involving either infidelity or a negotiated open relationship, such as polyamory.
Research from Africa has found that men who had polygamous marriages with multiple wives had higher levels of T than men in monogamous marriages. Van Anders' found that in her studies, men and women who reported that they were polyamorous, and involved in nonmonogamous relationships of one sort or another, all had higher levels of T. Both men and women who were polyamorous had higher levels of T than most other men and women.
This is very interesting research, as other findings show that when men and women get married, levels of T decrease. So, there's a greater likelihood that people with lower T will get married, or stay married, and their levels of T decrease even further when they get married. Unless they are in a relationship that involves sex or even the possibility of sex, with other people. For men, they don't have to actually have sex with other people for their T levels to remain high. Research by both van Anders, and by McIntyre, et al (2006) found that when a man is in a relationship where he COULD have sex with other people, either openly or through infidelity, his levels of T remain high. Men in casual sexual relationships with no commitment retain higher levels of T as well. Simply having a real interest in pursuing outside relationships leads to men maintaining higher levels of T.
Research with women, relationships and T is somewhat different though, in interesting ways. Women who are single tend to have higher levels of T. But, when they get married, or enter into a monogamous relationship, or even just enter into a casual sex relationship, their levels of T drop. But, when a woman identifies as polyamorous, her levels of T are higher than both partnered, and single women.
A really interesting thing in all this research is that levels of T are related not to actual sex with people, but in the interest in having sex with other people. Researchers call this mating effort, using the term to reflect the level of interest and thought that people put into pursuing sex with other people. In most monogamous relationships, that level of interest drops. But, in people with high T who tend towards infidelity, or who are in open relationships, that interest remains high, and keeps their T high, in a circular fashion. The T predicts that interest in sex with others will be high, AND the interest in sex with other people keeps the levels of T high, even when men and women enter into relationships where their levels of T would normally drop.
So, what does all this mean? Building evidence suggests that T has a very strong and interactive effect in the mating strategies and behaviors of human beings. Both before birth, and throughout a person's life, their levels of T increase the likelihood of them either engaging in infidelity, or at least planning to, or being in relationships where they CAN have sex with others, whether they do or not. Can you use this to predict whether your partner is thinking of cheating? Could people measure their partner's levels of T to guard against infidelity? Can you see that? "We'll, I know you're telling me that you don't want to cheat, but your levels of testosterone say you do!"
But, there are other things that affect T levels as well, including sports, involvement in political campaigns, and aggression. These things work within the physical and mental cycle of T in a complex and individualistic way, differently within each person. I do find it interesting though, that men in professions such as professional athletes, police, and politics all have higher levels of T, AND rates of infidelity are higher in these professions. These professions draw for men with higher levels of T, and their involvement in the activities of dominance, aggression and competition in the profession maintain higher levels of T. Similarly, those higher levels of T make infidelity more likely, and higher rates of infidelity keep those levels of T higher, all in a complex web of influences and effects, that is still being parsed apart.
